This invention relates to an automatic transmission control apparatus.
Conventionally, an automatic transmission has a torque convertor for receiving rotation output by the engine and a gearbox having planetary gear units, each made up of a plurality of gear elements. Gear-changing is effected based on the vehicle speed, throttle opening and the like in accordance with a preset gear-changing pattern.
In a conventional automatic transmission, ranges such as P (parking) range, R (reverse) range, N (neutral) range, D (drive) range, S (second) range and L (low) range can be selected.
For example when changing from the D range to the N range by operation of a shift lever, the rotation of the idling engine is transmitted through the torque convertor to the gearbox and a creep phenomenon may occur wherein the vehicle creeps forward even if the accelerator pedal is not being depressed.
This creep phenomenon is prevented by a neutral state achieved by a forward clutch, or first clutch, which is engaged when the gearbox is in a forward gear, being allowed to slip when a range for moving the vehicle forward (hereinafter referred to as a "forward range") such as the D range, the S range or the L range is selected, the accelerator pedal is released, the brake pedal is depressed, and it is detected that the vehicle speed is substantially "0".
With the engine idling during D range travel, the oil pressure supplied to the hydraulic servo of the first clutch is feedback-controlled and a state of slipping engagement is created so that the input-output speed differential of the torque convertor becomes a value predetermined as necessary to prevent the creep phenomenon from occurring (cf. U.S. Pat. No. 4,775,938).
In this kind of artificial neutral state (hereinafter called "the neutral control state"), for example when the accelerator pedal is depressed and the first clutch is reengaged, delay of engagement due to stroke loss of the first clutch piston, engine racing and engagement shock can be prevented.
However, in the prior art automatic transmission control referred to above, because the neutral control state is created by the first clutch being brought into a state of slipping engagement, some of the input torque from the engine is transmitted to the gearbox, thus subjecting the vehicle to idling vibration, diminishing fuel consumption and reducing the intended effectiveness of the artificial neutral state.
Also, because the first clutch slips as it transmits torque to its output side, the friction material of the first clutch is heated, thus reducing the useful service life of the friction material.